Rainfall

Dry conditions continued across much of NSW in October, with below to very much below average rainfall recorded in the southern half of the state, and about average rainfall recorded in the northern half. The statewide average rainfall was 26.4 mm, which is lower than the historical average1 of 48.9 mm. The Murray Darling Basin recorded an average of 26.3 mm, compared to the historical average of 44.1 mm.

The continued dry conditions across NSW are, in part, due to the states of both the Pacific and Indian Oceans over recent months. The late development (by historical standards) of a La Niña in the Pacific has resulted in a less than typical rainfall response across eastern Australia. Additionally, Australia's climate over recent months has been influenced by unusually cool ocean temperatures to the north and northwest of the continent - a state known as the 'Positive Dipole Mode' of the Indian Ocean Dipole.

Ocean temperatures in this region have been cooling since June, when historically, these waters would have been expected to warm, as the La Niña evolved in the Pacific. These cooler than normal waters (Positive Dipole Mode) inhibit the formation of northwest cloudbands, which are a major source of winter and spring rain for central and southeastern Australia during La Niña years. For more information about the Indian Ocean Dipole, please contact the NSW Climate Services Centre (details below).

Temperature

NSW experienced very warm days in October, with above to very much above average maximum (day-time) temperatures recorded over much of the state, particularly in the east. The statewide average maximum temperature was 27.6°C, which is 3.2°C above the historical average1 of 24.4 °C (records since 1950), making it the 2nd warmest October on record for statewide average maximum temperatures (current record for October is 27.7°C, set in 2006), and the 4th consecutive October with above average maximum temperatures.

The statewide average mean temperature (the average of the daily maximum and minimum temperature) was 19.5°C, which is 2.0°C above the historical average of 17.5°C, making this the equal warmest October on record for statewide average mean temperatures (equal to 19.5°C recorded in October 2006)

Statewide minimum (night-time) temperatures in October were close to average throughout the state, but were significantly cooler than day-time temperatures. The statewide average minimum temperature was 11.3°C, which is 0.8°C above the historical average of 10.5°C. The large variation between day-time and night-time temperatures in October resulted in a significant diurnal temperature range, with a statewide average diurnal temperature range of 16.3°C, which is 2.4°C above the historical average of 13.9°C. This makes it the 3rd warmest October on record for statewide average diurnal temperature ranges.

This distribution of temperatures across NSW can be attributed to several factors. In the current drought conditions, low soil and atmospheric moisture content enable the land surface to heat rapidly during the day and then cool quickly at night, as heat is radiated easily through clear dry night skies. Additionally, westerly winds throughout October brought warm, dry, continental air across to NSW from the central interior, resulting in warm, sunny days and clear, cooler nights.

Other phenomena

Severe thunderstorms affected parts of the northeast throughout October. On the 8th, 4 cm sized hail was reported in Grafton and Glen Innes, and on the 8th/9th, 2cm to 6cm sized hail was reported in Lismore. On the 26th a tornado was reported in Dunoon (north of Lismore), causing siginificant damage to local houses and buildings.

Notes

This statement has been prepared based on information available at
10 am on Thursday 1 November 2007.
Some checks have been made on the data, but it is possible that results will change
as new information becomes available.

A note about deciles: Deciles are used to give an element (in this case rainfall or temperature) a ranking. Deciles are calculated by arranging the totals in ascending order (from lowest to highest) then splitting them into 10 equal groups (thus the groups are called deciles). The first group would be in decile range one, the second group in decile range two, etc up to the highest annual totals (highest 10 per cent) being in decile range 10.

1Averages: Averages are based on the period 1961 to 1990 which is a convention of the World Meteorological Organisation

Normals are long-term averages based on observations from all available years of record, which vary widely from site to site. They are not shown for sites with less than 20 years of record for temperature and less than 30 years of record for rainfall, as they cannot then be calculated reliably.